The Phototrophic Community Found in Lake Khilganta (an Alkaline Saline Lake Located in the Southeastern Transbaikal Region)

The structure of the phototrophic community found in Lake Khilganta (the Agin-Buryat Autonomous Area), a shallow saline soda lake (depth, 35–45 cm; water mineralization, 45 g/l; alkalinity, 30 mg-equiv/l; pH 9.5) has been studied. The bottom of the lake is covered with a 10- to 15-mm microbial mat, whose basis is formed by the filamentous cyanobacterium Microcoleus chthonoplastes. The mat exhibits pronounced layering and contains a significant amount of minerals. Six zones, which have characteristic colors and consistencies and are composed of intermittent layers, have been identified along the vertical profile. Live phototrophic bacteria have been found in the three upper zones. The bulk of the cyanobacteria is concentrated in the upper zone. In the lower zones, the development of purple bacteria has been observed. The diurnal dynamics of the vertical distribution of phototrophic microorganisms, which results from variations in the physicochemical environmental parameters, is described. Ectothiorhodospira sp. are dominant among the anoxyphotobacteria present. Their number, determined according to the inoculation method, is 10⁶–10⁷ cells/ml. The purple bacteria of the genera Allochromatium, Thiocapsa, and Rhodovulum are also present. Experiments with isolated pure cultures have shown that the anoxygenic photosynthetic bacteria of Lake Khilganta are halotolerant and alkalitolerant or alkaliphilic. In liquid enrichment cultures, at pH 9.5, the ratio of anoxyphotobacteria species is close to that observed in the lake. When the pH is increased to 10.4, it is Ectothiorhodospira, which is the most adapted to life under increased mineralization and alkalinity, that predominantly develops. Photosynthetic activity has been observed in the three upper mat zones and constitutes, on average, 1.5 g C/(m²h); the share of anoxygenic photosynthesis accounts for 75–95% of the total productivity. The main role in sulfide oxidation belongs to the phototrophic anoxyphotobacteria and cyanobacteria. In terms of the physicochemical conditions and structure of the phototrophic community, Lake Khilganta is similar to shallow saline water bodies of marine origin. The main differences consist in the increased alkalinity and in the consequent prevalence of alkaliphilic and alkalitolerant microorganisms and in the absence of representatives of the neutrophilic group of green sulfur bacteria.__________Translated from Mikrobiologiya, Vol. 74, No. 3, 2005, pp. 410–419.Original Russian Text Copyright © 2005 by Kompantseva, Sorokin, Gorlenko, Namsaraev.     

Diversity of extremophilic purple phototrophic bacteria in Soap Lake, a Central Washington (USA) Soda Lake

Culture-based and culture-independent methods were used to explore the diversity of phototrophic purple bacteria in Soap Lake, a small meromictic soda lake in the western USA. Among soda lakes, Soap Lake is unusual because it consists of distinct upper and lower water bodies of vastly different salinities, and its deep waters contain up to 175 mM sulfide. From Soap Lake water new alkaliphilic purple sulfur bacteria of the families Chromatiaceae and Ectothiorhodospiraceae were cultured, and one purple non-sulfur bacterium was isolated. Comparative sequence analysis of pufM, a gene that encodes a key photosynthetic reaction centre protein universally found in purple bacteria, was used to measure the diversity of purple bacteria in Soap Lake. Denaturing gradient gel electrophoresis and subsequent phylogenetic analyses of pufMs amplified from Soap Lake water revealed that a significant diversity of purple bacteria inhabit this soda lake. Although close relatives of several of the pufM phylotypes obtained from cultured species could also be detected in Soap Lake water, several other more divergent pufM phylotypes were also detected. It is possible that Soap Lake purple bacteria are major contributors of organic matter into the ecosystem of this lake, especially in its extensive anoxic and sulfidic deep waters.     

Microbial communities of the stratified soda Lake Doroninskoe (Transbaikal region)

The physicochemical properties, species composition, and vertical distribution of microorganisms in the water column, shoreline microbial mat, and small shoreline mud volcanoes of the stratified soda Lake Doroninskoe were investigated in September 2007. The lake is located in the Transbaikal region, in the permafrost zone (51°25′N; 112°28′E). The maximal depth of the contemporary lake is about 6 m, the pH value of the water is 9.72, and the water mineralization in the near-bottom horizon is 32.3 g l⁻¹. In summer, the surface oxygen-containing horizon of the water column becomes demineralized to 26.5 g l⁻¹; at a depth of 3.5–4.0 m, an abrupt transition occurs to the aerobic zone containing hydrosulfide (up to 12.56 g l⁻¹). Hydrosulfide was also detected in trace quantities in the upper water horizons. The density stratification of the water column usually ensures stable anaerobic conditions until the freezing period (November and December). The primary production of oxygenic phototrophs reached 176–230 μg l⁻¹. High rates of dark CO₂ assimilation (61–240 μg l⁻¹) were detected in the chemocline. Within this zone, an alkaliphilic species of sulfur-oxidizing bacteria of the genus Thioalkalivibrio was detected (10⁴ cells ml⁻¹). Lithoheterotrophic bacteria Halomonas spp., as well as bacteriochlorophyll a-containing aerobic anoxygenic phototrophic bacteria (AAP) Roseinatronobacter sp. capable of thiosulfate oxidation, were isolated from samples collected from the aerobic zone (0–3 m). The water transparency in September was extremely low; therefore, no visible clusters of anoxygenic phototrophic bacteria (APBs) were detected at the boundary of the hydrosulfide layer. However, purple sulfur bacteria which, according to the results of the 16S rRNA gene analysis, belong to the species Thioalkalicoccus limnaeus, Ectothiorhodospira variabilis, “Ect. magna,” and Ect. shaposhnikovii, were isolated from samples of deep silt sediments. Ect. variabilis and Ect. shaposhnikovii were the major APB species in the shoreline algo-bacterial mat. The halotolerant bacterium Ect. shaposhnikovii, purple nonsulfur bacteria of the genus Rhodobacter, and AAP of Roseococcus sp. were isolated from the samples collected from mud volcanoes. All these species are alkaliphiles, moderate halophiles, or halotolerant microorganisms.     

Anoxygenic phototrophic bacteria from microbial communities of Goryachinsk thermal spring (Baikal Area,Russia)

Species composition of anoxygenic phototrophic bacteria in microbial mats of the Goryachinsk thermal spring was investigated along the temperature gradient. The spring belonging to nitrogenous alkaline hydrotherms is located at the shore of Lake Baikal 188 km north-east from Ulan-Ude. The water is of the sulfate-sodium type, contains trace amounts of sulfide, and salinity does not exceed 0.64 g/L, pH 9.5. The temperature at the outlet of the spring may reach 54°C. The cultures of filamentous anoxygenic phototrophic bacteria, nonsulfur and sulfur purple bacteria, and aerobic anoxygenic phototrophic bacteria were identified using the pufLM molecular marker. The fmoA marker was used for identification of green sulfur bacteria. Filamentous cyanobacteria predominated in the mats, with anoxygenic phototrophs comprising a minor component of the phototrophic communities. Thermophilic bacteria Chloroflexus aurantiacus were detected in the samples from both the thermophilic and mesophilic mats. Cultures of nonsulfur purple bacteria similar to Blastochloris sulfoviridis and Rhodomicrobium vannielii were isolated from the mats developed at high (50.6–49.4°C) and low temperatures (45–20°C). Purple sulfur bacteria Allochromatium sp. and Thiocapsa sp., as well as green sulfur bacteria Chlorobium sp., were revealed in low-temperature mats. Truly thermophilic purple and green sulfur bacteria were not found in the spring. Anoxygenic phototrophic bacteria found in the spring were typical of the sulfur communities, for which the sulfur cycle is mandatory. The presence of aerobic bacteriochlorophyll a-containing bacteria identified as Agrobacterium (Rhizobium) tumifaciens in the mesophilic (20°C) mat is of interest.     

Alkalispirillum mobile gen. nov., spec. nov., an alkaliphilic non-phototrophic member of the Ectothiorhodospiraceae

From cultures of the anoxygenic phototroph Halorhodospira halophila SL-1, an aerobic, gram-negative spirillum was isolated. This moderately halophilic, alkaliphilic bacterium was motile by means of a single polar flagellum. It is described here as Alkalispirillum mobile gen. nov., spec. nov. Phylogenetic analysis of the Alkalispirillum mobile 16S rRNA gene led to its classification in the gamma-subclass of the Proteobacteria, as it appears closely related to phototrophic purple sulfur bacteria of the genera Ectothiorhodospira and Halorhodospira. Surprisingly, A. mobile is an obligate aerobe. The organism grows optimally with a number of carboxylic acids (such as sodium acetate) as carbon source, at 2% (i.e. approximately 0.34 M) sodium chloride, at pH 9-10, and at temperatures ranging from 35 to 38 degrees C. The dominant cellular fatty acids of Alkalispirillum mobile are C12:0, C16:0, C18:1cis11, and C18:0; its G+C content is 66.2+/-0.5 mol%.     

Structure and development of a benthic marine microbial mat

Abstract Vertically stratified microbial communities of phototrophic bacteria in the upper intertidal zones of the North Sea island of Mellum were investigated. Growth and population dynamics of the cyanobacterial mat were followed over three successive years. It was concluded that the initial colonization of the sandy sediments was by the cyanobacterium Oscillatoria . In well-established mats, however, the dominant organism was Microcoleus chthonoplastes . The observed succession of cyanobacteria during mat development is correlated with nitrogen fixation. Nitrogen fixation is necessary in this low-nutrient environment to ensure colonization by mat-constructing cyanobacteria. Under certain conditions, a red layer of purple sulfur bacteria developed underneath the cyanobacterial mat in which Chromatium and Thiocapsa spp. dominated, but Thiopedia and Ectothiorhodospira spp. have also been observed. Measurements of light penetrating the cyanobacterial mat indicated that sufficient light is available for the photosynthetic growth of purple sulfur bacteria. Profiles of oxygen, sulfide and redox potential within the microbial mat were measured using microelectrodes. Maximum oxygen concentrations, measured at a depth of 0.7 mm, reached levels more than twice the normal air saturation. Dissolved sulfide was not detected by the microelectrodes. Determination of acid-distilled sulfide, however, revealed appreciable amounts of bound sulfide in the mat. Redox profiles measured in the mat led to the conclusion that the upper 10 mm of the sedimentary sequence is in a relatively oxidized state.     

Development of a real-time PCR method for the detection of fossil 16S rDNA fragments of phototrophic sulfur bacteria in the sediments of Lake Cadagno

Lake Cadagno is a crenogenic meromictic lake situated in the southern range of the Swiss Alps characterized by a compact chemocline that has been the object of many ecological studies. The population dynamics of phototrophic sulfur bacteria in the chemocline has been monitored since 1994 with molecular methods such as 16S rRNA gene clone library analysis. To reconstruct paleo-microbial community dynamics, we developed a quantitative real-time PCR methodology for specific detection of 16S rRNA gene sequences of purple and green sulfur bacteria populations from sediment samples. We detected fossil 16S rDNA of nine populations of phototrophic sulfur bacteria down to 9-m sediment depth, corresponding to about 9500 years of the lake's biogeological history. These results provide the first evidence for the presence of 16S rDNA of anoxygenic phototrophic bacteria in Holocene sediments of an alpine meromictic lake and indicate that the water column stratification and the bacterial plume were already present in Lake Cadagno thousands of years ago. The finding of Chlorobium clathratiforme remains in all the samples analyzed shows that this population, identified in the water column only in 2001, was already a part of the lake's biota in the past.     

Partitioning of CO2 Incorporation Among Planktonic Microbial Guilds and Estimation of In Situ Specific Growth Rates

Partitioning of CO₂ incorporation into oxygenic phototrophic, anoxygenic phototrophic, and chemolithoautotrophic guilds was determined in a freshwater lake (Lake Cisó, Banyoles, Spain). CO₂ incorporation into the different types of microorganisms was studied at different depths, during diel cycles, and throughout the year. During winter holomixis, the whole lake became anoxic and both the anoxygenic and chemolithoautotrophic guilds were more active at the surface of the lake, whereas the activity of the oxygenic guild was negligible. During stratification, the latter guild was more active in the upper metalimnion, whereas the anoxygenic guild was more active in the lower metalimnion. Specific growth rates and doubling times were estimated for the most conspicuous phototrophic microorganisms. Doubling times for Cryptomonas phaseolus ranged between 0.5 and 192 days, whereas purple sulfur bacteria (Chromatiaceae-like) ranged between 1.5 and 238 days. These growth rates were similar to those calculated with a different approach in previous papers and indicate slow-growing populations with very large biomass. Overall, the annual total CO₂ incorporation in Lake Cisó was 220 g C m⁻². Most of the CO₂ incorporation, however, was due to the chemolithoautotrophic guild (61% during holomixis and 56% during stratification), followed by the anoxygenic phototrophic guild (35 and 19%, respectively) and the oxygenic phototrophs (4 and 25%, respectively), making dark carbon fixation the key process in the autotrophic metabolism of the lake.     

Carbon isotope fractionation by anoxygenic phototrophic bacteria in euxinic Lake Cadagno

Anoxygenic phototrophic bacteria utilize ancient metabolic pathways to link sulfur and iron metabolism to the reduction of CO₂. In meromictic Lake Cadagno, Switzerland, both purple sulfur (PSB) and green sulfur anoxygenic phototrophic bacteria (GSB) dominate the chemocline community and drive the sulfur cycle. PSB and GSB fix carbon utilizing different enzymatic pathways and these fractionate C‐isotopes to different extents. Here, these differences in C‐isotope fractionation are used to constrain the relative input of various anoxygenic phototrophs to the bulk community C‐isotope signal in the chemocline. We sought to determine whether a distinct isotopic signature of GSB and PSB in the chemocline persists in the settling fraction and in the sediment. To answer these questions, we also sought investigated C‐isotope fractionation in the water column, settling material, and sediment of Lake Cadagno, compared these values to C‐isotope fractionation of isolated anoxygenic phototroph cultures, and took a mass balance approach to investigate relative contributions to the bulk fractionation signature. We found a large C‐isotope fractionation between dissolved inorganic carbon (DIC) and particulate organic carbon (POC) in the Lake Cadagno chemocline. This large fractionation between the DIC and POC was also found in culture experiments carried out with anoxygenic phototrophic bacteria isolated from the lake. In the Lake Cadagno chemocline, anoxygenic phototrophic bacteria controlled the bulk C‐isotope fractionation, but the influence of GSB and PSB differed with season. Furthermore, the contribution of PSB and GSB to bulk C‐isotope fractionation in the chemocline could be traced in the settling fraction and in the sediment. Taken together with other studies, such as lipid biomarker analyzes and investigations of other stratified lakes, these results offer a firmer understanding of diagenetic influences on bacterial biomass.     

Seasonal changes in the structure of the anoxygenic photosynthetic bacterial community in Lake Shunet, Khakassia

Seasonal studies of the anoxygenic phototrophic bacterial community of the water column of the saline eutrophic meromictic Lake Shunet (Khakassia) were performed in 2002 (June) and 2003 (February-March and August). From the redox zone down, the lake water was of dark green color. Green sulfur bacteria predominated in every season. The maximum number of green sulfur bacteria was 10(7) cells/ml in summer and 10(6) cells/ml in winter. A multi-syringe stratification sampler was applied for the study of the fine vertical distribution of phototrophs in August 2003; the sampling was performed every five centimeters. A five-centimeter-thick pink-colored water layer inhabited by purple sulfur bacteria was shown to be located above the layer of green bacteria. The species composition and ratio of purple bacterial species depended on the sampling depth and on the season. In summer, the number of purple sulfur bacteria in the layer of pink water was 1.6 x 10(8) cells/ml. Their number in winter was 3 x 10(5) cells/ml. In the upper oxygen-containing layer of the chemocline the cells of purple nonsulfur bacteria were detected in summer. The maximum number of nonsulfur purple bacteria, 5 x 10(2) cells/ml, was recorded in August 2003. According to the results of the phylogenetic analysis of pure cultures of the isolated phototrophic bacteria, which were based on 16S rDNA sequencing, green sulfur bacteria were close to Prosthecochloris vibrioformis, purple sulfur bacteria, to Thiocapsa and Halochromatium species, and purple nonsulfur bacteria, to Rhodovulum euryhalinum and Pinkicyclus mahoneyensis.     

Anoxygenic Photosynthesis and Nitrogen Fixation by a Microbial Mat Community in a Bahamian Hypersaline Lagoon

Simultaneous measurements of photosynthesis (both oxygenic and anoxygenic) and N(inf2) fixation were conducted to discern the relationships between photosynthesis, N(inf2) fixation, and environmental factors potentially regulating these processes in microbial mats in a tropical hypersaline lagoon (Salt Pond, San Salvador Island, Bahamas). Major photoautotrophs included cyanobacteria, purple phototrophic bacteria, and diatoms. Chemosystematic photopigments were used as indicators of the relative abundance of mat phototrophs. Experimental manipulations consisted of light and dark incubations of intact mat samples exposed to the photosystem II inhibitor DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], a dissolved organic carbon source (D-glucose), and normal seawater (37(permil)). Photosynthetic rates were measured by both O(inf2) and (sup14)C methods, and nitrogenase activity (NA) was estimated by the acetylene reduction assay. Moderate reductions in salinity (from 74 to 37(permil)) had no measurable effect on photosynthesis, O(inf2) consumption, or NA. CO(inf2) fixation in DCMU-amended samples was (symbl)25% of that in the control (nonamended) samples and demonstrated photosynthetic activity by anoxygenic phototrophs. NA in DCMU-amended samples, which was consistently higher (by a factor of 2 to 3) than the other (light and dark) treatments, was also attributed to purple phototrophic bacteria. The ecological implication is that N(inf2) fixation by anoxygenic phototrophs (purple phototrophic bacteria and possibly cyanobacteria) may be regulated by the activity of oxygenic phototrophs (cyanobacteria and diatoms). Consortial interactions that enhance the physiological plasticity of the mat community may be a key for optimizing production, N(inf2) fixation, and persistence in these extreme environments.     

Heterotrophic dinitrogen fixation (acetylene reduction) in phosphate-fertilised Microcoleus chthonoplastes microbial mat from the hypersaline inland lake ‘la Salada de Chiprana’ (NE Spain)

Microcoleus chthonoplastes dominated microbial mats are conspicuous along the shallow littoral zone in Lake Chiprana, a hypersaline lake located in the Ebro river basin in north-eastern Spain. Pigment data show that these mats included diatom species and anoxygenic phototrophs, Chloroflexus-type bacteria and purple bacteria. In situ, these mats showed low rates of dinitrogen fixation (acetylene reduction). Acetylene reduction was stimulated about 30-fold in excised mats after moderate phosphate fertilisation during 2 weeks incubation in a mesocosm. Pigment analyses showed that this treatment had little impact on the phototrophic community structure, except that it induced a decrease of Chloroflexus-type bacteria. The use of metabolic inhibitors indicated that methanogenic archaea and aerobic heterotrophic bacteria were the major dinitrogen fixers in this system. This is in agreement with the fact that the mat-building cyanobacterium M. chthonoplastes lacks the dinitrogenase reductase nifH gene and with the fact that acetylene reduction rates were strongly stimulated by additions of H₂/CO₂, methanol, fructose and sucrose, but not by lactate, acetate, formate and glucose. No significant differences where found for acetylene reduction rates when comparing light and dark incubations of these microbial mats. However, acetylene reduction rates were enhanced in the light when the near infrared (NIR) light was filtered out, which arrested anoxygenic photosynthesis. We suggest, therefore, that the chemoheterotrophic dinitrogen fixing bacteria were in competition with anoxygenic phototrophic bacteria for organic substrates, while the latter did not contribute to dinitrogen fixation in the mat.     

Anoxygenic phototrophic bacteria of the high-altitude meromictic Lake Gek-Gel,Azerbaijan

The anoxygenic phototrophic bacterial community of the high-altitude meromictic Lake Gek-Gel (Azerbaijan) was investigated in September 2003. The highest concentration of bacteriochlorophyll e (48 μg/l) was detected at a depth of 30 m; the peak of bacteriochlorophyll a (4.5 μg/l) occurred at 29 m. Phylogenetic analysis revealed that brown-colored green sulfur bacteria Chlorobium phaeobacteroides predominated in the lake. Nonsulfur purple bacteria phylogenetically close to Blastochloris sulfoviridis were found in insignificant amounts; these organisms have not been previously reported in Lake Gek-Gel.     

Pigments, light penetration, and photosynthetic activity in the multi-layered microbial mats of Great Sippewissett Salt Marsh, Massachusetts

The multi-layered microbial mats in the sand flats of Great Sippewissett Salt Marsh were found to have five distinct layers of phototrophic organisms. The top 1–3 mm contained oxygenic phototrophs. The lower 3–4 mm contained anoxygenic phototrophic bacteria. The uppermost gold layer contained diatoms and cyanobacteria, and chlorophyll a was the major chlorophyll. The next layer down was green and was composed of primarily filamentous cyanobacteria containing chlorophyll a. This was followed by a bright pink layer of bacteriochlorophyll b-containing purple sulfur bacteria. The lowest layer was a thin dull green layer of green sulfur bacteria containing bacteriochlorophyll c. The distribution of the chlorophylls with depth revealed that two-thirds of the total chlorophyll in the mat was composed of bacteriochlorophylls present in the anoxygenic phototrophys. The cyanobacterial layers and both purple sulfur bacterial layers had photoautotrophic activity. Light was attenuated in the uppermost layers so that less than 5% of the total radiation at the surface penetrated to the layers of anoxygenic phototrophys.     

Benthic phototrophic community from Kiran soda lake,south-eastern Siberia

Phototrophic bacterial mats from Kiran soda lake (south-eastern Siberia) were studied using integrated approach including analysis of the ion composition of water, pigments composition, bacterial diversity and the vertical distribution of phototrophic microorganisms in the mats. Bacterial diversity was investigated using microscopic examination, 16S rRNA gene Illumina sequencing and culturing methods. The mats were formed as a result of decomposition of sedimented planktonic microorganisms, among which cyanobacteria of the genus Arthrospira predominated. Cyanobacteria were the largest part of phototrophs in the mats, but anoxygenic phototrophs were significant fraction. The prevailing species of the anoxygenic phototrophic bacteria are typical for soda lakes. The mats harbored aerobic anoxygenic phototrophic bacteria, purple sulfur and non-sulfur bacteria, as well as new filamentous phototrophic Chloroflexi. New strains of Thiocapsa sp. Kir-1, Ectothiorhodospira sp. Kir-2 and Kir-4, Thiorhodospira sp. Kir-3 and novel phototrophic Chloroflexi bacterium Kir15-3F were isolated and identified.

     

Community Shift from Phototrophic to Chemotrophic Sulfide Oxidation following Anoxic Holomixis in a Stratified Seawater Lake

Most stratified sulfidic holomictic lakes become oxygenated after annual turnover. In contrast, Lake Rogoznica, on the eastern Adriatic coast, has been observed to undergo a period of water column anoxia after water layer mixing and establishment of holomictic conditions. Although Lake Rogoznica''s chemistry and hydrography have been studied extensively, it is unclear how the microbial communities typically inhabiting the oxic epilimnion and a sulfidic hypolimnion respond to such a drastic shift in redox conditions. We investigated the impact of anoxic holomixis on microbial diversity and microbially mediated sulfur cycling in Lake Rogoznica with an array of culture-independent microbiological methods. Our data suggest a tight coupling between the lake''s chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic sulfur bacteria were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event, the anoxygenic phototrophic sulfur bacteria entirely disappeared, and the homogeneous, anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers related to the GSO/SUP05 clade. This study is the first report of a community shift from phototrophic to chemotrophic sulfide oxidizers as a response to anoxic holomictic conditions in a seasonally stratified seawater lake.